How Do Surface Defects Change Local Wettability of the Hydrophilic ZnS Surface? Insights into Sphalerite Flotation from Density Functional Theory Calculations

In the flotation process, surface–reagent reactions occur on water precovered surfaces; therefore, investigation on the mineral–water interactions is essential in such systems. In the present study, the importance of surface vacancies was scrutinized by simulating water–surface complexes up to multilayer water molecules on the defective ZnS (110) surface using density functional theory. We restricted our attention to the ZnS surfaces with single sulfur, single zinc, and double neighboring congruent vacancies. Our simulation results revealed that the absence of Zn or S atoms on the surface layer has a different effect on the local surface wettability: while removing a S atom increases the hydrophobicity, removing a Zn atom increases the hydrophilicity around the defect site. The presence of double neighboring congruent vacancies further stabilized the water adsorption, whereas, by removing one Zn atom and one S atom, respectively, from surface upper and downward hills, water was dissociatively adsorbed on the ZnS surface. However, increasing the number of water molecules in a system altered the adsorption configuration and energies, indicating that considering an isolated water molecule cannot reflect the real surface circumstances at the water–surface interface. In general, in the ZnS system, surface reconstruction occurs through molecular adsorption of water on the surface by forming the Zn–O bond. Our surface analysis revealed that the level of hydrophilicity of the ZnS surface is higher when the number of undercoordinated S atoms on the surface is increased.